The present invention relates to automatic flight control for an aircraft and more particularly to a method and device for detecting noise on a guide signal of LOC type received by an aircraft.
There exist systems for assisting a pilot in landing airplanes under poor visibility conditions. Such systems are commonly known as ILS (Instrument Landing System).
An ILS system is generally composed of a ground portion and of a portion on board the airplanes. The ground portion generally comprises radio-frequency transmitters that permit establishment of an imaginary axis of approach to the runway by means of a horizontal radio beam and a vertical radio beam.
The horizontal radio beam, known as Localizer or LOC, establishes the axis of the runway, while the vertical radio beam, known as Guide Slope or GS, establishes the slope of descent of the airplane to the edge of the runway. The LOC emits a VHF (Very High Frequency) signal in the 108-118 MHz frequency band. The GS emits a UHF (Ultra High Frequency) signal in the 329-335 MHz frequency band. Thus the LOC signal is used to determine a difference between the axis of displacement of the airplane and the runway axis, and the GS signal is used to determine a difference between the axis of displacement of the airplane and the nominal approach slope. The LOC and GS beams are narrow and sensitive to perturbations.
Different incidents encountered by the airline companies or during flight tests undertaken by the manufacturers reveal perturbations of the LOC beam. Particular consequences of such perturbations for the automatic flight controls are untimely alarms about excessive deviation, premature engagement in an LOC capture mode, more or less large excursions of the airplane parameters (such as slip angle or yaw rate, close to the ground if the automatic pilot is already in LOC beam holding mode, or an excursion in lateral trajectory if the airplane is in automatic phase of rolling on the ground.
These perturbations can occur in very diverse and sometimes unpredictable situations. For example, these perturbations can occur during an undetected and uncorrected breakdown of the LOC receiver, during an undetected breakdown of an LOC transmitter or during perturbations of the transmitter, especially when an airplane flies over the LOC transmitter on takeoff or stops in front of the LOC transmitter on the runway. In all of these cases, signal reflections cause perturbations while the airplane in automatic landing mode can be at low altitude or on the ground.
The problem is accentuated by the diversity of noise profiles (multiple frequency) and by the fact that the airline companies are tending to generalize the use of automatic landing, even in good visibility.